DNA molecules contain internucleotide phosphodiester linkages which are degraded by exonucleases present in cells, culture media and human serum. For example, degradation by exonucleases in tissue culture media of DNA may be observed within about 30 minutes to about six hours. Synthetic oligodeoxynucleotides with phosphodiester linkages are routinely used in genetic engineering, for example, to locate specific RNA or DNA fragments from a library. The long-term stability of an oligonucleotide for this utility is not a major concern, since the oligonucleotide is usually not exposed to the relatively stringent environment of the culture medium, therefore exonuclease degradation is not a substantial problem.
However, it is in fact frequently desirable to produce oligodeoxynucleotides which are stable (i.e., for more than several hours or days) for long-term uses. For example, a oligodeoxynucleotide with phosphodiester linkages can be used to block protein synthesis by hydrogen bonding to complementary messenger RNA thereby providing a tool for use in an antisense fashion. Exonuclease-stable oligodeoxynucleotides could also be utilized to form triple-helix DNA which would interfere with the transcription process or with DNA replication, by competing with naturally occurring binding factors or by gene destruction. However, in order to utilize synthetic oligonucleotides in this manner, they must be stable to exonucleases, the major activity of which in cells and serum appears to be 3' to 5', i.e., digestion of oligonucleotides begins starting at the 3' end.
The present invention is accordingly directed to such exonuclease-stable oligonucleotides.